Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an x-ray tube incorporating a spiral groove bearing (SGB) therein.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. The anode structure is typically supported by ball bearings and is rotated for the purpose of distributing the heat generated at a focal spot. Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the ball bearings.
A liquid metal bearing may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation as is commonly understood in the art. Gallium and alloys thereof are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
Gallium tends to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable. As such, a refractory metal such as molybdenum is typically used as the base material for an SGB, and spiral grooves are typically machined in the surface, as known in the art, in order to provide a pumping action to maintain the liquid metal in its desired location. Not only is such material resistant to corrosion, but it tends to be vacuum-compatible and thus lends itself to an x-ray tube application. However, one concern that may be encountered in the use of a liquid metal is that of ensuring adequate wetability of bearing surfaces with the liquid metal. When adequate wetability does not occur, the liquid metal does not completely fill the SGB and the SGB may not uniformly distribute the liquid metal throughout the gap during use, thus shortening the life of the x-ray tube.
Wetability may be negatively affected due to exposure of the base metal to air or moisture prior to and/or during assembly, causing an oxide layer to form thereon. The oxide layer, in turn, deteriorates the wetability of the surface of the part with the liquid metal. Known techniques have been employed to improve or maintain the wetability of the base material under these circumstances. One known technique includes firing the bearing surfaces at approximately 800° C. in hydrogen and then storing the parts in a oxygen-protective atmosphere, like nitrogen or argon, until use. Another known technique includes coating the bearing parts with a carbide, boride, or nitride using, for instance, a physical vapor deposition (PVD) technique.
Another known technique includes applying molybdenum as a diffusion barrier using PVD. However, although molybdenum may be employed when applying such a diffusion barrier using PVD, the base material of the diffusion barrier is typically identical to the base material. Alternatively, materials applied via PVD using materials that differ from the base material tend to be limited to 2000 nm thicknesses for proper application in order to avoid cracking due to thermal mismatch of the applied barrier and the base metal. The thermal mismatch may be mitigated to an extent by employing a coating having an expansion coefficient that is similar to the base metal. However, such solutions tend to limit the number of base metal/coating options. Further, because of the thickness limitation, such materials are precluded from post-machining, thus necessitating that the diffusion barrier be applied having thicknesses that fall within the desired final tolerances of the final part. Also, because of the thickness limitation, such solutions to improve wetability still necessitate that the base material be resistive to the corrosive effects of the liquid metal, such as molybdenum. However, molybdenum tends to be expensive, both as a base material, and in terms of machining and processing.
One technique for minimizing base material expense and improving functionality is to include the preferred base metal (i.e., molybdenum) only in regions that will contact liquid metal. An extension made of a less expensive material may then be brazed or otherwise attached thereto, the extension serving as a mechanical connection as support for an anode. In other words, as an example, a stationary center shaft may support a rotatable support structure having an anode attached thereto. The center shaft may be made entirely of the preferred base metal, or the cost thereof may be reduced by attaching a less expensive steel thereto via a braze or other attachment method, thus reducing the total amount of the preferred base metal. Such a design may result in cost savings because of the less expensive steel portion being used in lieu of the preferred base metal. However, cost savings achieved while using this technique are typically offset to an extent by the additional attachment processing, such as by attaching the extension thereto having a hermetic seal.
One drawback in the use of molybdenum is that molybdenum can form an intermetallic layer with gallium that is not stable at typical operating temperatures of an SGB. Thus, an intermetallic layer tends to form as a result of contact between a solid molybdenum surface and liquid gallium, acting as an abrasive if it tends to break down, or particulate, on contact between stationary and rotating parts, which can lead to early life failure of the SGB. Formation of the intermetallic layer is a function of temperature and follows Arrhenius aging principles as is known in the art.
Thus, an SGB may be built of molybdenum and having gallium as a liquid metal therein, and with proper handling and processing a SGB made as such may provide adequate performance for the life of the x-ray tube. However, a base metal of molybdenum tends to be costly, and an alternative SGB having a molybdenum coating only in regions of contact with gallium typically includes a costly braze step. Machining of molybdenum or a molybdenum coating includes additional costs as well, and an additional wetting step (i.e., firing in a hydrogen environment) is a costly processing step associated with a molybdenum-based SGB. Further, molybdenum forms an intermetallic that is unstable at typical operating temperatures and, as imaging applications tend toward an increase in power, operating temperatures likewise increase, thus accelerating the growth and formation of the molybdenum-gallium intermetallic layer.
Therefore, it would be desirable to have an apparatus and method that reduces total costs associated with fabricating and using an SGB.